By: Jim Baty
Codes are developed to provide assurance that structures are designed and constructed to withstand the test of time not only for life safety but also for long-term durability. Why is it then that codes can also be so complex that the very implementation of them causes such struggles to occur that work often is delayed or, worse yet, halted indefinitely? Residential construction is no different in this way than any other construction market today. Recognizing this, the American Concrete Institute created a committee specific to the residential industry (332-Residential Concrete) with the goal of creating guides and standards to help shape the quality and efficiency of this market.
Nearly 20 years after the formation of this committee, the first ACI Residential Concrete Code has been published. This article continues to digest some of the more substantive areas altering or improving the requirements for residential concrete as the industry prepares to shift from design and construction by ACI 318, chapter 22 to ACI 332.
The ACI 332 Standard, developed by this committee provides the minimum cast-in-place design and construction requirements for one- and two-family dwellings and their accessory structures. The current version of the code (332-04) specifically addresses those requirements for concrete footings (wall, thickened slab, and isolated), basement or foundation walls constructed only with removable forms and slabs-on-ground. Other elements and methods for residential construction do exist but are beyond the scope of this current document. Those not included remain under the provisions of ACI 318 and other code references.
Aspects of some codes are developed to circumvent common performance problems that arise from decisions made during construction or during service. One area that fits this description in the new ACI 332 Standard is slabs-on-ground. Perhaps one of the most frequent topics for serviceability questions received at the headquarters for the Concrete Foundations Association (CFA) as well as organizations like the American Society of Concrete Contractors (ASCC) pertains to the development of cracks and surface deterioration in concrete slabs. Among many slab characteristics, the provisions of ACI 332-04 establish conditional requirements for the design and construction of slabs-on-ground that are more stringent than those currently enforced throughout most of the North American regions.
The material requirements for concrete in general can be found in chapter 4 of ACI 332-04. Although provisions exist for mix designs applied to all residential concrete elements, the requirements from this chapter largely affect the mix design for slabs-on-ground more than any other. Recognizing that exposure to chemicals is largely based on surface area in these concrete elements, the departure from other code documents on mix design requirements can be readily seen in Table 4.1, where minimum 28-day compressive strengths and maximum slumps are found:
The issue of weather probability refers to the severity of freezing and thawing conditions to exist, which also result in the increased use of deicing chemicals for these Type 3 conditions. To determine the locations affected by each level of weather probability, a map (figure 4.1) has been provided that also departs from the maps defined in current code references.
Figure 4.1 - Location of Weathering Regions Source: ASTM International, C33-03 Standard Specification for Concrete Aggregates
Severe weather probability is the category that has received the most attention. This is largely due to the amount of weather- and chemical-related failures that have occurred in the regions defined by this category. A "severe" classification, as stated in 332-04, is:
where weather conditions encourage or require the use of deicing chemicals or where there is potential for a continuous presence of moisture during frequent cycles of freezing and thawing.
Determining the slump for these regions is equally as important as the 28-day compressive strength. Slump, as referenced by Table 4.1, refers to the characteristics of the specified mix design based on the water to cementitious material ratio. This is not intended to limit the workability of the mix as mid-range and high-range water reducers can be applied to improve flowability and workability. However, specific language limits the application of water to any mixture beyond this specified ratio limitation due to the impact it has on the long-term durability for the surface characteristics of the slabs.
Beyond compressive strength and slump, provisions also exist for the air content based on the maximum aggregate size, the use of calcium chloride (or restriction of use) and the minimum cover that must be maintained both above and below any steel reinforcement in a slab-on-ground.
In addition to specifying minimum requirements for the determination of the mixture design, 332-04 establishes minimum design characteristics for the slab-on-ground elements themselves. Turning to chapter 8, readers will find provisions addressing such aspects as the design of the sub-grade support, minimum thickness of the slab, spacing of joints and minimum reinforcement requirements among others.
Serviceability for slabs-on-ground begin with proper support and proper thickness. Without a quality base including proper drainage and without adequate thickness for both load capacity and reinforcement cover, the performance of the slab-on-ground can be severely impacted. ACI 332 states:
Slabs-on-ground shall be continuously supported on undisturbed soil or with fill and base …
Where fill and a base are required, 332 provides:
The fill shall be compacted to provide uniform support of the slab and shall be free of organic and foreign material. Fill depths shall not exceed 24 in. for clean sand or gravel and 8 in. for earth.
A 4 in.-thick base course consisting of clean graded sand, gravel, crushed stone, or crushed blast-furnace slag passing a 2 in. sieve shall be placed on the prepared subgrade when the slab is below grade.
By providing a base of undisturbed soil (with all organics removed) or a uniformly compacted aggregate or earth base, the slab can effectively distribute applied loads without concern for differential settlement or local cracking failures assuming the thickness of the slab is adequate. Providing for slab thickness, 332-04 states:
The minimum thickness of slabs-on ground shall be 3.5-in.
There are conditions where this thickness may need to be greater than this specified thickness but with a minimum of 3.5 inches, no less than minimum cover requirements can be maintained on both sides of the required steel area of 0.5 percent the slab cross-sectional area (0.0175 square inches for a 3.5-inch minimum slab). Cover over reinforcement must be maintained at 0.75-inch for interior slabs and 1.5 inches for exterior slabs (garage slabs included) with the reinforcement located in the upper half of the slab depth. In order to achieve these requirements, 332-04 also requires this reinforcement to be supported in this designated position during concrete placement.
The thickness of the slab also impacts the shrinkage crack control that must be provided in the form of joints. Joints are created in slabs-on-ground by one of three methods. They are 1) Formed where a concrete placement ends; 2) Tooled immediately after the slab surface is troweled; or 3) Sawed using an early-entry or wet-saw method. The spacing for these joints is controlled by Table 8.1 of 332-04 as shown below:
The application of joint spacing requirements is based on the fact that shrinkage occurs in slabs at an approximate rate of 5/8 inch per 100 feet. In order to be effective, the joint must be provided in the slab a minimum of 1/4 of the slab thickness.
Finally, ACI 332-04 provides practical recognition of common construction detailing that results in slabs-on-ground and footings cast monolithically at conditions around a slab perimeter (common in shallow frost-depth regions) or at locations within a slab such as isolated pad footings or load-bearing walls. Figures 6.3 and 6.4 below are included in the standard as a reference for these allowable conditions.
Figure 6.3 - Exterior unformed thickened slab footing
Figure 6.4 - Interior unformed thickened slab footing
Issues that are not addressed by the Standard can be found in the ACI 332 Guide that will be updated within the next year. One of the primary items that contractors and home owners should be aware of is the application of deicing chemicals. Although the concrete requirements for slabs have been strengthened through this document, the exposure of slabs and other exterior concrete elements to these chemicals until after one winter season should be prohibited. Caution should always be taken when applying any chemical, solution or particulate to a new concrete slab to maintain the desired surface for the life of the element.
Through the combination of strengthened requirements and recognition of practical issues for constructability based on empirical data, ACI 332-04 has provided a standard reference for improved performances of slab-on-ground conditions in all residential markets.
Jim Baty is Technical Director of the Concrete Foundations Association.